Yes, you can get the factory heater-only set up that came on cars that weren't equipped with a/c.
Unfortunately, since it seems about 95% of G-bodies came with a/c, complete set ups are getting difficult to find.
I swapped the non a/c boxes into both my 442 and my Cutlass wagon.
It's a little more involved than just the box on the firewall, and for it to work correctly, you'll have to replace every single piece of the system, as nothing is shared with the a/c set-ups.
There are also aftermarket conversions available that require an extensive amount of fabrication for very little pay-off, or you can just modify your existing box.